Electrophotographic processing is commonly performed by the following procedures: a latent electrostatic image is formed by various electrical means on a photoreceptor made of selenium or some other suitable photoconductive material; and toner particles are deposited on the latent image by a suitable method of development, such as magnetic brush method, so as to produce a visible image.
In the development step, carrier particles are used in order to impart an appropriate amount of positive or negative electrical charges to the toner. Carriers are generally divided into two types: "coated" and "uncoated". For various reasons including the life of developer used, the former type has a better performance and various kinds of "coated" carriers have so far been developed and used commercially.
While coated carriers are required to possess various characteristics, particularly important ones include appropriateness of the polarity of charges generated by electrification, high impact and wear resistance, good adhesion between the core and the coating layer, and uniformity of charge distribution. In these respects, the prior art coated carriers still have problems to be solved and an ideal product has yet to become known. Fluorine-based resins, e.g., fluorinated vinyl polymers used as coating materials as described in U.S. Pat. Nos. 3,922,382 and 3,798,167 have high resistance to surface soiling due to their low surface energy. However, they are located at an extremely negative side in the triboelectric series and are liable to be charged negatively so that they cannot charge a toner negatively, and in addition they are unable to provide strong adhesion to the magnetic core. Acrylic polymers as described in Japanese Patent Application (OPI) No. 13954/72 are satisfactory in terms of mechanical strength, adhesion to the core and positive chargeability, but, on the other hand, they are generally considered to be unsatisfactory with respect to their resistance to surface soiling. In this way, the prior art coated carriers have had to employ coating materials that have their own advantages and disadvantages.